XNAExtras build 0916 release

This release includes a bunch of small enhancements and bug fixes. The only major class addition is XSpriteGroup, which allows you to manipulate a group of sprites as a single unit.

From the release notes:

   (XML Schema) Added XML schema definition (XSD) files for bitmap and texture files
   (BMFontGen) Fixed bug where custom glyphs couldn't replace chars that didn't exist in font
   (BitmapFont) Added Dispose method
   (BitmapFont) Fixed TextBox to format the string only once
   (BitmapFont) Allow fonts to be loaded from embedded resources
   (BitmapFont/XSpriteManager) Changed GetXMLAttribute to not throw exceptions
   (XSpriteManager) Changed sprites to sort by order of creation when at the same depth
   (XSpriteBase) Added Visible property
   (XSprite) Added SetTexture method
   (XSprite) Expose sprite alpha value directly
   (TexturePool) Fixed bug in AddTexture(string,Color) where the texture wasn't being created if the GraphicsDevice already existed

Thanks to everyone (and especially to David Zakar (DMZ), redwyre and Martin Szinger) who took the time to send comments, bug reports and feature requests.

[Update: Oops! Björn points out that I forgot the link to the updated version. The link to the latest version is here.]